Power transfer apparatus for concurrently transmitting data and power over data wires

ABSTRACT

Power supply current, sufficient to power a remote network device, is transmitted concurrently with a network data signal over a transmission line. A power-sourcing network device that can include a coupling circuit provides power and data to the remote network device. The coupling circuit can also be included in a stand-alone device. The remote network device (which can be a wireless access point) can separate the power signal from the data signal and use the power supply current to further process or retransmit the data signal. The power signal may be a low frequency relative to the frequency of the data signal, or it may be DC.

RELATED APPLICATIONS

This Application is a continuation of application Ser. No. 09/974,237,filed Oct. 10, 2001, now U.S. Pat. No. 6,496,105; which is acontinuation of Ser. No. 09/675,730, filed Sep. 29, 2000, now U.S. Pat.No. 6,329,906, issued Dec. 11, 2001; which is a continuation ofapplication Ser. No. 09/416,067, filed Oct. 12, 1999, now U.S. Pat. No.6,140,911, issued Oct. 31, 2000; which is a continuation of applicationSer. No. 08/865,016, filed May 29, 1997, now U.S. Pat. No. 5,994,998,issued Nov. 30, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to the field of data networking andcommunications, and in particular to interconnecting computers to alocal area network (“LAN”) or a wide area network (“WAN”) through datalines that also carry power.

2. Description of the Related Art

Network devices typically communicate via wired data lines and receivepower from a separate line. For example, personal computers (“PCs”) maycommunicate Ethernet signals via category three (CAT-3) or category five(CAT-5) twisted pair wire and receive power from a second cableconnected to a power source, such as a wall socket or a battery.However, it is desirable to be able to eliminate the need for the secondcable. The following describes examples of network devices that benefitfrom the elimination of the separate power line, and then describes someof the inadequacies of previous solutions.

Plain old telephone service (“POTS”) combines a voice signal with apower signal. The combined signal is transmitted over twisted pair cablebetween the telephone and the line card at the public telephone exchangeoffice. The line card also supplies power over the two wires carryingthe voice signal. However, the voice signal supported by POTS is notsufficient for bandwidth intensive communications needs, such as,Ethernet communications. Similarly, ISDN communications transmit powerand digital data between an ISDN modem and a telephone switch. However,ISDN data rates are more than an order of magnitude lower than Ethernetdata rates.

Wireless network adapters can interconnect PCs, or other networkeddevice. The wireless network adaptors use, for example, infrared (IR) orradio frequency (RF) modulation to transmit data between wireless accesspoints and the wireless adaptors connected to PCs. Although the wirelessadaptors and wireless access points may be more expensive thancomparable wired equipment, they provide savings in wiring costs andpermit greater flexibility by allowing the PCs to be moved to anylocation within the range of the system without the necessity ofrewiring the building.

Typically, a transceiver (meaning transmitter and receiver) called awireless access point, mounted at an elevated location, such as on aceiling or high on a wall, provides network data communications betweena network hub, switch, router or server, to all the PCs located in thatroom which are equipped with a compatible wireless networking adaptor.The wireless access point is an active electronic device that requires acommunications link to a hub or server as well as electrical power tooperate. Both the data signal and power signal must be provided to thewireless access point. The data signal is typically at a lower voltagethan the power signal, but at a significantly higher frequency,sufficient to sustain a high data transfer rate (e.g., 100 kilobits persecond or higher). The available power is usually 110V or 220V AC atfrequencies below one hundred Hz. Often two separate sets of wires areused to carry the data signal and power signal. One set of wires is usedto couple the wireless access point and the hub and the other set ofwires is used to couple the wireless access point to the power outlet.

Eliminating the need for separate power and data wiring simplifies theinstallation of a wireless access point and can reduce the cost of theinstallation. Therefore, it is desirable to transmit sufficientelectrical power to operate the wireless access point through thenetwork cable that is used to connect the wireless access point to thehub or server.

One possible solution is to transmit power on the unused wires of thedata cable. An example of this approach can be found in the VIPSLAN-10™product manufactured by the JVC Information Products Company of Irvine,Calif. Of course this requires that additional, unused wire pairs beavailable in the data cable, which may not always be available. Also, ifa change in the networking standard in the future dictates the use ofthe currently unused wire pairs in the networking cable, this solutionbecomes difficult to implement.

Therefore, what is needed is a solution that reduces the wiringrequirements to transmit data and power to a wireless access pointwithout having to use additional wire pairs.

SUMMARY OF THE INVENTION

One embodiment of the invention includes an apparatus for providingelectric power supply current to a network device across a transmissionline. A power and data coupler (“the coupler”) is coupled to one end ofthe transmission line. The transmission line is also adapted fortransmission of a data signal. The coupler has a data input and a powerinput. Power supply current from the power input is coupled to datasignal from the data input and the combined power supply current anddata signal is coupled to one end of the transmission line. The oppositeend of the transmission line is coupled to a power and data decoupler(“the decoupler”). The decoupler has a power output and a data output.Both the data output and power output of the decoupler are coupled tothe network device. The combined power supply current and data signal isdecoupled by the decoupler, and the data signal is supplied to the dataoutput and the power supply current is supplied to the power output.Thus, the data signal and the power supply current are coupled andtransmitted via the transmission line from the coupler to the decouplerand then decoupled and provided separately to the network device.

In another embodiment, the transmission line includes two transmissionlines. One of the transmission lines carries both data and powersignals.

In other embodiments, the power signal includes alternating currentand/or direct current.

In another embodiment, the transmission lines include twisted paircables.

In other embodiments, the network devices include wireless accesspoints, network interface cards, peripheral devices and/or networkcomputers.

These features of the invention will be apparent from the followingdescription which should be read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overview of an installation of a power transfer apparatus;

FIG. 2 is an overview of a power transfer apparatus for use withwireless access points;

FIG. 3 is a schematic diagram of a power transfer apparatus;

FIG. 4 is a more detailed schematic drawing showing a DC power transferapparatus and corresponding circuitry located in the wireless accesspoint; and

FIG. 5 is a more detailed schematic drawing showing an AC power transferapparatus and corresponding circuitry located in the wireless accesspoint. This apparatus provides electrical isolation to the wirelessaccess point.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following describes multiple embodiments of the invention. In oneembodiment, power and data are combined and transmitted to a networkdevice such as a wireless access point. The wireless access point usesthe power signal to power communication circuits for communicating withwireless network nodes. Because the power and data are combined, theinstallation of the wireless access point is simplified and may reducethe cost of installing the wireless access points.

Power Transfer Apparatus Overview

FIG. 1 shows the overall configuration of one embodiment of theinvention including a power transfer apparatus. The following lists theelements in FIG. 1 and then describes those elements.

FIG. 1 includes the following elements: an external power source 150; apower cable 120; a data cable 130; a power and data coupler 110; anetwork cable 160; a power and data decoupler 170; and, a network device100.

The following describes the coupling of the elements of FIG. 1. Theexternal power source 150 couples to the power and data coupler 110 viathe power cable 120. The power cable 120 couples to the power and datacoupler 110. The communications network 140 couples to the data cable130. The data cable 130 couples to the power and data coupler 110. Thepower and data coupler 110 also couples to the network cable 160. Thenetwork cable 160 couples to the power and data decoupler 170. The powerand data decoupler 170 couples to the network device 100.

The following describes the elements in greater detail and thendescribes how the elements act together.

The external power source 150 provides a power signal 105 to the powerand data coupler 110. Various embodiments of the invention use differentexternal power sources 150: such as, a computer's power supply, abattery, or a wall outlet and adaptor. What is important, however, isthat there is some source of power that can eventually be supplied tothe network device 100.

In one embodiment, the power cable 120 is a standard two wire powercable. Other embodiments use other power transfer apparatuses to providepower to the power and data coupler 110.

The communications network 140 is representative of many different typesof communications networks supported by various embodiments of theinvention. Example communications networks 140 include FDDI, Ethernet(including ten Mbits/s, one hundred Mbits/s, and one gigibits/sstandards), ATM, token ring, and AppleTalk. However, what is importantis that a data signal 104 is communicated between the communicationnetwork 140 and the network device 100.

The power and data coupler 110 couples the power signal 105 with thedata signal 104 to produce a combined power and data signal 107. Thepower and data coupler 110 is described in greater detail below. What isimportant is that there is some combined power and data signal 107 thatcan eventually be supplied to the network device 100.

The network cable 160 includes one or more wires for transmitting thecombined power and data signal 107. In one embodiment, the network cable160 includes an CAT-3, CAT-5 twisted pair cable, or coaxial cable.

The network device 100 represents a class of devices supported byvarious embodiments of the invention. For example, in one embodiment,the network device 100 includes a wireless access point. In anotherembodiment, the network device 100 includes a personal computer having anetwork interface card. In another embodiment, the network device 100includes a network computer.

The following describes the general operation of the elements of FIG. 1.A data signal is communicated to the power and data coupler 110 via thedata cable 130 from a communications network 140. The combined power anddata signal 107 is transmitted over the network cable 160 to the networkdevice 100. In this embodiment, the network cable 160 is longer thanthree meters and the combined power and data signal 107 communicatesdata at greater than one megabit/second. (In another embodiment, thenetwork cable length conforms to the IEEE 802.3 specification.) Thus,the power and data coupler 110 supplies both power and data to thenetwork device 100. The network device 100 uses the power to operatewhich includes receiving, processing, and generating the data signal.

Wireless Access Point having a Power Transfer Apparatus

FIG. 2 is an overview of a power transfer apparatus for use withwireless access points. The following lists the elements in FIG. 2 andthen describes those elements. FIG. 2 includes: an external power source150, a power adaptor 256, a power cable 120, a hub 240, a data cable130, a power and data coupler 110, a network cable 160, a wirelessaccess point 200, and a number of remote nodes. The remote nodes includelaptop computers 280 and a desktop computer 270. Each computer includesa wireless adaptor card 295.

The power adaptor 256 steps down available electrical power from 117 or220 volts AC to an AC or DC voltage that is high enough to provideadequate voltage for the wireless access point 200. In one embodiment,the power adaptor 256 supplies an output voltage of approximatelytwenty-four volts. Other embodiments of the invention have other outputvoltages, such as thirty-six and forty-eight volts. The power adaptor256 is described in greater detail in the description of FIG. 5.

The hub 240 is not needed in one embodiment of the invention to supplythe data signal. Therefore, in other embodiments of the invention, thedata signal is supplied by a network computer, a router, and a bridge.In one embodiment, the hub 240 provides an Ethernet based data signalsupporting a data transfer rate of at least one megabit/second.

Regarding the power and data coupler 110, what is important is thatthere is some combined power and data signal 107 that can eventually besupplied to the wireless access point 200. Therefore, for example, inone embodiment, the power and data coupler 110 is included in a networkcard in the hub 240. The power signal 105, taken from the hub's powersupply, can then be combined with the data signal provided by the hub240.

The wireless access point 200 is an example of a network device 100. Thewireless access point 200 includes a transceiver for providing wirelesscommunications with the wireless adaptor cards 295. In this example, thewireless access point 200 is mounted on the ceiling. The wireless accesspoint 200 is described in greater detail below.

The wireless adaptor cards 295 also include a transceiver forcommunicating with the wireless access point 200.

The desktop computer 270 and the laptop computer 280 are examples ofsome devices that may be included in one embodiment of the invention.For example, the desktop computer 270 can include an IBM compatiblepersonal computer, or a MacOS™ compatible computer. However, otherembodiments of the invention include other remote network nodes such asa Newton™ personal digital assistant and a pager.

The following describes the general operating of the system shown inFIG. 2. The power adapter 256 supplies power to the power and datacoupler 110 while the hub 240 provides a data signal to the power anddata coupler 110. The power and data coupler 110 communicates a combinedpower and data signal 107 to the wireless access point 200. The wirelessaccess point 200 is powered from the power part of the power and datasignal 107. The wireless access point 200 communicates a wireless datasignal with the wireless adapter cards 295. The wireless data signalcorresponds to the data signal from the hub 240. The wireless adaptercards 295 provide the desktop computer 270 and the laptop computers 280with the wireless data signal.

Schematic Diagram of a Power Transfer Apparatus

FIG. 3 is a schematic diagram of a power transfer apparatus. Thefollowing first lists the elements in FIG. 3, then describes theelements' couplings, and then describes the elements' interactions.

FIG. 3 includes: the power cable 120, the data cable 130, power and datacoupler 110, the network cable 160, and the wireless access point 200.The power and data coupler 110 includes a coupler power input port 320,a coupler data port 380 and a coupler port 360. The wireless accesspoint 200 includes a power and data decoupler 170 and a network accesspoint 307. The power and data decoupler 170 includes a decoupler port365, a decoupler power output port 325 and a decoupler data port 335.

The elements of FIG. 3 are coupled as follows. The power cable 120 iscoupled to the coupler power input port 320. The data cable 130 iscoupled to the coupler data port 380. The network cable 160 is coupledto the coupler port 360 and to the decoupler port 365. The wirelessaccess point 200 is coupled to the decoupler power output port 325 andto the decoupler data port 335.

The power and data decoupler 170 performs a function similar to thatperformed by the power and data coupler 110. However, the power and datadecoupler 170 decouples the power signal from the data signal. The powerand data decoupler 170 can then supply the power signal to the networkaccess point 307 separately from the data signal.

The network access point 307 includes the transceiver for communicatingwith the remote nodes.

The elements of FIG. 3 interact as follows. The power cable 120 providespower supply current to the coupler power input port 320. The data cable130 transmits the network data signal to the coupler data port 380. Thepower and data coupler 110 combines the power signal and the data signaland outputs this signal at the coupler port 360. The combined power anddata signal is transmitted on the network cable 160. The wireless accesspoint 200 receives the combined power and data signal through thedecoupler port 365. The power and data decoupler 170 separates thenetwork data signal from the power supply current. The power and datadecoupler 170 then supplies the power signal at the decoupler poweroutput port 325 and communicates the data signal to the network accesspoint 307 at the decoupler data port 335. The network access point 307uses the power signal to power wireless data signals to the remotenodes. The wireless data signals correspond to the data signalcommunicated with the decoupler data port 335.

In another embodiment of the invention, separate transmit and receivepaths are supported between the power and data coupler 110 and the powerand data decoupler 170. In this embodiment, the data cable 130 includesat least two wires supporting a transmit path and two wires supporting areceive path. Note that power is only coupled to the transmit path wiresin one embodiment. While in another embodiment, all four wires are usedin the power transmission.

FIG. 4 shows a more detailed schematic of one configuration of thisinvention. The example shown in FIG. 4 is specifically adapted for the10Base-T twisted pair networking protocol. Other embodiments of theinvention support other network protocols. These embodiments includemodifications for the number of wires used by the particular networkprotocol. The following lists the elements of FIG. 4, describes theirinterconnections, and then describes the operation of the elements.

FIG. 4 includes: the power adapter 256, the power cable 120, the datacable 130, the network cable 160 and the wireless access point 200. Thepower adapter 256 includes a stepdown transformer 451, a diode bridge453, and a capacitor 455. The power and data coupler 110 includes: thecoupler data port 380, a pair of isolation transformers (isolationtransformer 412 and isolation transformer 413), a pair of center tappedinductors (inductor 416 and inductor 417), a pair of capacitors(capacitor 414 and capacitor 415), a pair of inductors (inductor 418 andinductor 419), a light emitting diode (LED 402), a resistor 403, and thecoupler power and data port 360. The wireless access point 200 includesthe network access point 307 and the power and data decoupler 170. Thepower and data decoupler 170 includes: the decoupler power and data port365, a pair of inductors (inductor 422 and inductor 423), a pair orcenter tapped inductors (inductor 524 and inductor 425), a pair ofcommon mode chokes (choke 426 and choke 427), a pair of capacitors(capacitor 428 and capacitor 429), a pair of isolation transformers(transformer 432 and transformer 433), a receive filter 434, a transmitfilter 435, a DC—DC converter 410, a decoupler power output port 325,and the decoupler data port 335. In one embodiment, the lowpass filters,the common mode choke, and the transformers are all part of the wirelessaccess point.

The elements in the power adapter 256 are coupled as follows. Theprimary winding of the transformer 451 is coupled to receive the powersignal from the power adapter 256. The diode bridge 453 is connected tothe secondary winding of the transformer 451. The capacitor 455 isconnected across the output of the diode bridge 453. The output of thediode bridge 453 is connected to power cable 120.

The elements in the power and data coupler 110 are coupled as follows.In this example, the data signal is carried on four wires. Thus, thecoupler data port 380 includes a four wire connection to the data cable130. The primary windings of the transformer 412 are connected to thetwo data input wires of the coupler data port 380. Similarly, theprimary windings of the transformer 413 are connected to the two dataoutput wires of the coupler data port 380. The capacitor 414 and thecapacitor 415 are connected in series with the secondary windings of thetransformer 412 and the transformer 413, respectively. The center tappedinductor 416 and two output data wires of the coupler output port 360are coupled across the secondary winding of the isolation transformer412. Similarly, the center tapped inductor 417 and two input data wiresof the coupler input port 360 are coupled across the secondary windingof the isolation transformer 413. The inductor 418 is connected betweenthe center tap of the inductor 416 and to the positive wires of thepower cable 120. The inductor 419 is connected between the center tap ofthe inductor 417 and the negative wires of the power cable 120. Theresistor 403 and LED 402 are connected across the positive and negativewires of the power cable 120.

The elements in the wireless access point 200 are coupled as follows.The center tapped inductor 422 and the center tapped inductor 423connect across the two input wires and two output wires, respectively,of the decoupler port 365. The inductor 422 connects to the center tapof the center tapped inductor 424 and to the positive terminal of theDC—DC converter 410. Similarly, the inductor 423 connects to the centertap of the center tapped inductor 425 and to the negative terminal ofthe DC—DC converter 410. The choke 426 connects to the ends of thecenter tapped inductor 424 and across the primary winding of thetransformer 432. The choke 427 connects to the ends of the center tappedinductor 425 and across the primary winding of the transformer 433. Thereceive filter 434 connects between the secondary winding of thetransformer 432 and the two output wires of the decoupler port 335. Thetransmit filter 435 connects between the secondary winding of thetransformer 433 and the two input wires of the decoupler port 335. TheDC—DC converter 410 connects to the decoupler power output 325.

The power adapter 256 operates as follows. Power is received from theexternal power supply at the primary winding of the transformer 451. Thetransformer 451 electrically isolates the power adapter 256. The diodebridge 453 performs full wave rectification of the alternating currentfrom the secondary winding of the transformer 451. The capacitor 455helps in the full wave rectification to create a DC output. The windingratio of the transformer 451 and the value of the capacitor 455 isselected to provide the proper voltage output given the input voltageconnected to the primary of the transformer 451. The power adapter 256is representative of a variety of commercially available power adapters.

The power and data coupler 110 operates as follows. There is oneisolation transformer (e.g., transformer 412) and one center-tappedinductor (e.g., 416) for each pair of networking data wires used in theparticular networking standard. The data signal passes through thesetransformers with minimal loss. The transformers eliminate ground loopsbetween the power and data coupler 110 and any network devices attachedto coupler data port 330. The isolation transformers also isolate thepower and data coupler 110 in case of accidental contact between thedata cable 130 and a high voltage source. In one embodiment, theisolation transformer 412 and the isolation transformer 413 have awinding ratio of approximately 1:1 and an isolation of one thousand fivehundred volts. The capacitor 414 and the capacitor 415 remove DC currentfrom the data signal.

Each center-tapped inductor (e.g., inductor 416) presents an impedanceclose to zero Ohms for DC or low frequency AC current, however, theimpedance across each wire pair to the data signal is significantlyhigher. (The low frequency AC current is low relative to the data signalfrequency. In one embodiment, the low frequency AC current is less thanone hundred Hertz while the data signal is greater than one Megahertz.)The use of center-tapped inductors permits the current to flowrelatively unimpeded and balanced down each wire of the wire pairsconnected across the winding of each center-tapped inductor. The equalcurrent flow reduces the line resistance to DC and permits the currentto flow equally to/from each end of the center-tapped inductor. Theequal flow creates an equal and opposite DC flux within the core of thecenter-tapped inductor, preventing the saturation of the core of thecenter-tapped inductor. In one embodiment of the invention, the seriesinductor 418 and the series inductor 419 provide additional isolationbetween the power signal and the high-frequency data signal. The seriesinductors 418 and 419 are optional in some embodiments.

The data signal connection to the data cable 130 is provided throughcoupler data port 330 which is selected for compatibility with theparticular network protocol used. Certain data cables have wires thatare not used for data communication with certain protocols. For example,the CAT-3 or CAT-5 cable has four wires that are not used with the10BASE-T standard (i.e. two sets of pairs). The power transmissionapparatus of the invention transmits the power signal using only thewires normally used for data communication. The unused wires are notused.

One embodiment of the invention includes the resister 403 and the LED402. The LED 402 indicates whether the power signal is being received bythe power and data coupler 110. Although this indication is desirablefrom an operational point of view, the LED 402 and resistor 403 are notrequired for the operation of one embodiment of the invention.

The wireless access point 200 operates as follows. The wireless accesspoint 200 receives the combined power and data signal at the decouplerport 365. The DC, or AC power, flows through the center-tap of thecenter-tapped inductor 424 and the center-tapped inductor 425. The DC—DCconverter 410 is preferred because of its high efficiency and lowself-power dissipation (the DC—DC converter 410 allows for lower inputvoltages). However other devices, such as linear regulators, may be usedto regulated the specific voltage and varying current loads required bythe network access point 307. The series inductor 422 and the seriesinductor 423 enhance the isolation between the data and power lines. Thecommon mode choke 426 and the common mode choke 427 help suppress highfrequency signal components that cause electromagnetic interference withthe network access point 307. The data signal is provided across thesecondary windings of the isolation transformer 432 and the isolationtransformer 433. The data signal being sent to the network access point307 is then filtered using the receive filter 434. The data signal beingsent from the network access point 307 is filtered before being sent outon the network cable 160. The network access point 307 can then use thepower signal from the DC—DC converter 410 and communicate information toand from the remote nodes and the network using the data signal.

FIG. 5 shows an alternate embodiment of the invention. In thisembodiment, the power adapter 256 has been modified so that thesecondary winding of transformer 451 is directly coupled to the powercable 120. The power and data decoupler 170 includes the following newelements: a transformer 552, a diode bridge 554, and a capacitor 556.The primary winding of the transformer 552 is connected across to theinductor 422 and the inductor 423. The input of the diode bridge 554 iscoupled across the secondary winding of the transformer 552 and outputof the diode bridge 554 is coupled to the DC—DC converter 410. Thecapacitor 556 is connected across the output of diode bridge 554.

In the alternative embodiment of the invention, the power adapter 256provides low voltage AC power, instead of DC power, to the power anddata coupler 110. The transformer 551 has a winding ratio to create lowvoltage AC power from the input high voltage AC power. The low voltageAC power is combined, in the same manner described above for the DCpower, with the data signal. The combined power and data signal is thentransmitted via network cable 160. The low voltage AC power is separatedin the power and data decoupler 170 in the same manner as describedabove for the DC power. The low voltage AC power is then passed throughthe transformer 552 and the rectifying circuit (including the diodebridge 554 and the capacitor 556). The output of the rectifying circuitconnects to the DC—DC converter 410. This configuration provides furtherenhanced isolation to the data cable and any networking accessoriesconnected to the power and data coupler 110.

In one embodiment, the frequency of the AC power signal is substantiallyless than the frequency of the data signal. In various embodiments, theAC power signal has a frequency of 60 Hz, 440 Hz, and 56 Hz, while thedata signal has a frequency of approximately 1 MHz, 10 MHz, and 1 GHz.However, the exact frequencies are not important, only that there issome difference between the frequencies.

The preceding has described multiple embodiments of the invention. Inone embodiment, power and data are combined and transmitted to awireless access point. The wireless access point uses the power tocommunicate with wireless network nodes. Because the power and data arecombined, the installation of the wireless access point is simplifiedand may reduce the cost of installing the wireless access points.

While the foregoing invention has been described in referenced to someof its embodiments, it should be understood that various modificationsand alterations will occur to those practiced in the art. Suchmodifications and alterations are intended to fall within the scope ofthe appended claims.

We claim:
 1. A network device for transmitting both data signals andpower supply current over a transmission line, the network devicecomprising: a power input; and a coupling circuit that couples a datasignal between the network device and the transmission line, thecoupling circuit comprising: at least one inductor for coupling powersupply current from the power input to the transmission line.
 2. Thenetwork device of claim 1, wherein the at least one inductor comprises acenter-tapped inductor.
 3. The network device of claim 1, wherein thecoupling circuit further comprises at least one isolation transformerfor coupling data between the transmission line and the network device;wherein the at least one isolation transformer includes a data inputside and a transmission line side; and wherein the coupling circuitincludes at least one capacitor connected in series with thetransmission line side of the at least one isolation transformer.
 4. Thenetwork device of claim 3, wherein the transmission line side of the atleast one isolation transformer includes partial windings and the atleast one capacitor is connected between the partial windings.
 5. Thenetwork device of claim 2, wherein the coupling circuit furthercomprises at least one series inductor, the at least one series inductorbeing connected between the power input and a center of the at least onecenter-tapped inductor.
 6. A network device for transmitting both datasignals and power supply current over a transmission line, the networkdevice comprising: a power input; and a coupling circuit that couples adata signal between the network device and the transmission line, thecoupling circuit also provides power supply current from the power inputto the transmission line, the coupling circuit comprising: two isolationtransformers for coupling data between the network device and thetransmission line; and two center-tapped inductors for coupling powersupply current to the transmission line; wherein the two isolationtransformers each have a data input side and a transmission line side.7. The network device of claim 6, wherein the coupling circuit furthercomprises two capacitors, each capacitor being connected in series withthe transmission line side of a different one of the two isolationtransformers.
 8. The network device of claim 7, wherein each of the twocapacitors is connected between partial windings on the transmissionline side of a different one of the two isolation transformers.
 9. Thenetwork device of claim 6, wherein the coupling circuit furthercomprises two series inductors, each series inductor being connectedbetween the power input and a center of a different one of the twocenter-tapped inductors.
 10. The network device of claim 6, wherein eachcenter-tapped inductor is connected across the transmission line side ofa different one of the two isolation transformers.
 11. A couplingcircuit for coupling a data signal and power supply current to atransmission line for use in a system for providing electrical powersupply current to at least one network device, the coupling circuitcomprising: a data input and a power input, wherein the coupling circuitcouples power supply current from the power input and couples the datasignal from the data input to the transmission line; at least onecenter-tapped inductor for coupling the power supply current to thetransmission line; and at least one isolation transformer for couplingthe data input to the transmission line; wherein the at least oneisolation transformer includes a data input side and a transmission lineside; and wherein the coupling circuit includes at least one capacitorconnected in series with the transmission line side of the at least oneisolation transformer.